Solenoids are sometimes used in "hostile" interior environments within certain devices. An example of a hostile interior environment is the inside of the body of a solenoid valve which controls pressurized hydraulic fluid. Internal pressures as high as several thousand psi are often encountered. The solenoid must be constructed to withstand the rigors of such usage by continuing to operate properly over its lifetime, and it must also remain sealed to the valve body so that fluid does not leak past the solenoid to the valve's exterior. It is also vital for the solenoid to remain unaffected by the high pressure fluid environment.
A typical coil previously used in high pressure solenoid valves involves the winding of magnet wire onto a plastic bobbin. The construction may include some sort of metal sleeve or an extremely thick walled bobbin for strengthening the coil against the large hoop stresses the solenoid may experience due to its exposure to high pressure hydraulic fluid. These prior solenoid coils are also typically encapsulated to protect their windings from the hydraulic fluid. Disadvantages of prior solenoid coils include the following.
When a coil winding and bobbin sub-assembly is encapsulated, the encapsulant may fail to attain a hermetic seal because it does not adequately bond to the bobbin material. This creates voids that are actual or potential leak paths for the hydraulic fluid. Leakage of fluid to the winding can result in the magnet wire coating being attacked by the hydraulic fluid and becoming short-circuited. During the encapsulation process, the bobbin is subjected to the heat and pressure of molding, and the thermally and mechanically induced stresses can cause distortion and/or cracking of the bobbin. When a metal sleeve is used to strengthen the solenoid against hoop forces and/or to isolate a non-metallic bobbin from the hydraulic fluid, the sleeve itself may introduce a shorted turn effect that increases eddy current losses and therefore reduces the performance capabilities of the solenoid. The use of both a sleeve and a bobbin in combination with a coil, and the encapsulation thereof in a sub-assembly, tends to increase the package size of a solenoid contrary to the demands of the market for smaller size solenoids. If a coil is made by precision winding or layer winding magnet wire on a bobbin, very little hoop strength can be expected or gained from the winding itself because air gaps exist and they allow the turns to move: during the encapsulation process; when the coil is subjected to thermal changes during operation; and when the solenoid is subjected to high pressure usage. The use of a metal sleeve poses a manufacturing problem because it is cost-prohibitive to pinch trim both ends of the sleeve to size, and as a result it becomes more feasible to pinch trim one end and machine a chamfer on the other that will allow the O-ring seals that are needed for high-pressure sealing of the solenoid to the valve body to be inserted without damage.
The present invention relates to a new, unique, and cost-effective method of making a high-pressure solenoid coil that will exhibit those characteristics necessary for successful high pressure usage. One of those is a hermetically sealed coil. The specific methodology will be disclosed in the ensuing description which is accompanied by drawings. The disclosure presents a presently preferred embodiment in accordance with the best mode contemplated at the present time for carrying out the invention. Additional features and advantages may also be perceived by the reader as the disclosure proceeds.